1. Field of the Invention
The present invention relates to electrically conductive silicone rubber compositions having improved adhesion, which compositions are suitable for use in fields that require high electrical conductivity, for example, as current contact parts in electrical components and mobile electrical units, and sealants in electromagnetic shields, office equipment rollers, antistatic members, and connectors.
2. Prior Art
In the fields of electrical components and mobile electrical units requiring high electrical conductivity in their current contact portions and electromagnetic shields or the like requiring high electrical conductivity in their seals, such parts are often formed of silicone rubber compositions loaded with high-conductivity fillers. Where bond to resins or metals is additionally required, an adhesive primer must be applied to the bond surface before the rubber is molded thereto.
Undesirably, many primers contain solvents which give rise to environmental and working problems and also prolong the working time. Also, to lower the electrical resistance of silicone rubber, a large amount of highly conductive filler must be added. This increases the surface area of filler exposed on the rubber surface, with a possibility that mere application of the primer fails to provide sufficient adhesion.
An object of the invention is to provide an electrically conductive silicone rubber composition which cures into a part having a stable high conductivity and improved self adhesiveness.
It has been found that by blending an organopolysiloxane having aliphatic unsaturated groups with finely divided silica, a metal powder or an electrically conductive powder in the form of metal-plated inorganic filler or resin particles, and an adhesive aid, there is obtained an electrically conductive silicone rubber composition which cures into a part having a low volumetric resistivity, a stable high conductivity and improved self adhesiveness and is thus suitable as electrically conductive parts and electrical contacts in electrical equipment.
The invention provides an electrically conductive silicone rubber composition having improved adhesion, comprising
(A) 100 parts by weight of an organopolysiloxane having at least two aliphatic unsaturated groups in a molecule,
(B) 0.1 to 100 parts by weight of finely divided silica,
(C) 30 to 700 parts by weight of a metal powder or an electrically conductive metal-plated powder,
(D) 0.1 to 20 parts by weight of an adhesive aid, and
(E) a curing agent in an amount sufficient to cure the organopolysiloxane.
Also contemplated herein is an article comprising a substrate of a metal or resin and a coating of the conductive silicone rubber composition applied and cured to the substrate without a primer, the cured coating of the composition being integrated with the substrate.
Component (A) of the electrically conductive silicone rubber composition of the invention is an organopolysiloxane at least two aliphatic unsaturated groups, especially alkenyl groups in a molecule. Preferably, the organopolysiloxane has the average constitutional formula (1):
xe2x80x83R1aSiO(4xe2x88x92a)/2xe2x80x83xe2x80x83(1)
wherein R1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 12 carbon atoms, 0.001 to 20 mol % of the R1 groups are alkenyl groups, and xe2x80x9caxe2x80x9d is a positive number from 1.5 to 2.8.
In the alkenyl-bearing organopolysiloxane of formula (1), exemplary R1 groups are substituted or unsubstituted monovalent hydrocarbon groups having 1 to 12 carbon atoms, and preferably 1 to 8 carbon atoms, including alkyl groups such as methyl, ethyl, propyl, isopropyl, isobutyl, butyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and decyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl, cyclohexenyl, and octenyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; and halo- and cyano-substituted hydrocarbon groups such as chloromethyl, bromoethyl, 3,3,3-trifluoropropyl, 3-chloropropyl and cyanoethyl.
The substituent groups may be the same or different. The preferred content of alkenyl groups is 0.001 to 20 mol %, especially 0.01 to 10 mol % of the R1 groups. As noted above, it is essential for the organopolysiloxane to have at least two alkenyl groups in a molecule. The R1 groups may be any of the above-exemplified ones although the preferred alkenyl group is vinyl and the preferred other substituent groups are methyl and phenyl. In formula (1), xe2x80x9caxe2x80x9d stands for a positive number from 1.5 to 2.8, and preferably from 1.8 to 2.9.
The organopolysiloxane of formula (1) may have a linear molecular structure or a branched molecular structure including R1SiO3/2units and SiO4/2 units. Preferably it is a linear diorganopolysiloxane having a backbone that is basically composed of recurring R12SiO2/2 diorganosiloxane units, and which is capped at both ends of the molecular chain with R13SiO1/2 triorganosiloxy groups. Most preferably, the diorganopolysiloxane is capped with trivinylsilyl, divinylmethylsilyl or vinyldimethylsilyl groups at both ends of the molecular chain.
The alkenyl groups in the molecule may be bonded to silicon atoms either at the ends of the molecular chain or at the middle of the chain, or both. The presence of alkenyl groups bonded to at least the silicon atoms at both ends of the molecular chain is preferable in terms of the cure of the composition and the cured physical properties.
The alkenyl-bearing organopolysiloxane preferably has an average degree of polymerization in the range of about 100 to 3,000, especially about 200 to 2,000 when the composition is liquid, and in the range of about 3,000 to 100,000, especially about 4,000 to 20,000 when the composition is of millable type.
The alkenyl-bearing organopolysiloxane may be prepared by known methods. According to one suitable method, preparation may involve carrying out equilibrium reaction between an organopolysiloxane and a hexaorganodisiloxane in the presence of an alkali or acid catalyst.
Component (B) of the silicone rubber composition is finely divided silica. Its type is not critical and any silica powder that is used in conventional silicone rubber compositions, with the exception of metal-plated silica powder, may be employed. Illustrative examples include precipitated silica, fumed silica and fired silica having a specific surface area of at least 50 m2/g, and especially 50 to 400 m2/g, as measured by the BET method. Other suitable examples include ground quartz and diatomaceous earth having an average particle size of up to 50 xcexcm, and preferably within a range of 0.1 to 20 xcexcm.
The silica powder may be used directly without modification. Alternatively, the silica powder may be used in the form of a hydrophobic silica powder after surface treatment with a silazane such as hexamethyldisilazane, a silane such as trimethylchlorosilane or an organosilicon compound such as polymethylsiloxane. The silica powder may also be rendered hydrophobic by blending it with a surface treatment agent during compounding.
The finely divided silica (B) is incorporated in an amount of 1 to 100 parts by weight, and preferably 2 to 50 parts by weight, per 100 parts by weight of the organopolysiloxane (A). With less than 1 part of silica, the mechanical strength of the cured product may become low. The use of more than 100 parts of silica tends to obstruct proper loading of the conductive filler (C), adversely affecting the workability of the composition.
Component (C) is a metal powder or a conductive metal-plated powder (conductive powder having a metal plating surface layer) whose purpose is to confer the silicone rubber composition of the invention with electrical conductivity.
No limitations are imposed on the particle size of the metal powder, although an average particle size within a range of 0.05 to 100 xcexcm, and especially 0.1 to 10 xcexcm, is preferred.
The metal powder may have any suitable particle shape, including granular, dendritic or flake-like, or may be of irregular shape. Alternatively, a mixture of metal powders having a combination of these shapes may be used. To form a silicone rubber of low electrical resistance, it is desirable for the metal particles to be partially linked rather than dispersed in a completely discrete state.
Illustrative examples of metal powders that may be used in the invention include silver powder, gold powder and nickel powder, of which silver powder is especially preferred.
Any silver powder may be used without particular limitation, including those produced by electrolysis, grinding, heat treatment, atomization or chemical processes.
Any suitable known apparatus may be used to produce ground silver powder, including a stamp mill, ball mill, vibratory mill, hammer mill, pressure rollers or mortar and pestle. Nor is there any particular limitation on the conditions for rolling silver powder composed of reduced silver, atomized silver, electrolytic silver or mixtures of two or more thereof. Rather, it is essential to select such conditions according to such considerations as the particle size and shape of the silver powder to be used.
The conductive metal-plated powders that can be used herein as component (C) include metal-plated inorganic fillers and organic resin powders, for example, silica particles, glass beads, phenolic resin particles, and (meth)acrylic resin particles which are plated with gold or silver. Of these, metal-plated silica powder is preferred. Metal-plated silica powder consists of silica particles surfaces of which have been coated by metal plating. Examples of suitable plating metals include gold, silver and nickel, of which gold and nickel are especially preferred.
Preferably the metal-plated silica powder has a specific surface of at most 1 m2/g. A surface area greater than 1 m2/g may result in poor dispersibility when the powder is added to the silicone rubber composition.
The metal-plated silica powder preferably has a construction comprising silica covered with a nickel layer which in turn is covered with a gold layer. A four-layer construction of silica/silicon compound/nickel/gold in which a silicon compound lies between the silica and the nickel is especially advantageous for improving adhesion between the metal and the silica. Preferred examples of the silicon compound include carbon-functional (CF) silane monomers such as those made by Shin-Etsu Chemical Co., Ltd. under the trade name designations KBM-603, KBM-903, KBE-603 and KBE-903, and silicon-based polymeric compounds with reducing properties.
The method of producing the metal-plated silica powder is not subject to any particular limitation. A typical example of production includes the following steps.
(1) A first step in which silica powder is treated with a silicon compound, preferably one with reducing properties, to form a layer of the silicon compound on the surface of the silica.
(2) A second step in which the powder resulting from the first step is treated with a solution containing a metal salt composed of metal having a standard redox potential of at least 0.54 to induce the deposition of a colloid of the metal onto the silicon compound layer that has been formed over the silica surface.
(3) A third step in which electroless nickel plating is carried out using the above metal colloid as the catalyst so as to form a layer of metallic nickel on the surface of the silicon compound layer.
(4) A fourth step in which gold plating is carried out, thereby forming a layer of gold on the metallic nickel layer. These four steps are described more fully below.
The silica starting material used to form the metal-plated silica powder is a powder composed of silicon dioxide, and thus has a high heat resistance. The particles making up the silica starting material may have any suitable shape, such as granular, fibrous or flake-like, although a spherical shape, because it has the smallest specific surface area for a given particle size, is desirable for minimizing the amount of plating metal (typically nickel or gold) used and for achieving high loading in the silicone rubber composition. Such silica can be easily prepared by such methods as burning chlorosilane, hydrolyzing alkoxysilane, oxidizing gasified metallic silicon or fusing silica powder. To lower the specific surface area, it is desirable for the silica particles to have no interior cavities that communicate with the particle surface. In this sense, the use of fused silica is especially preferred. The silica powder has an average particle size in a range of 0.01 to 1,000 xcexcm, and preferably 0.1 to 100 xcexcm. At less than 0.01 xcexcm, the specific surface area becomes large and may require the use of such a large amount of plating metal as to be prohibitively expensive. On the other hand, at a particle size greater than 1,000 xcexcm, the metal-plated silica powder may be difficult to mix into the silicone rubber composition.
Production of the metal-plated silica powder according to the invention is preferably carried out by treating the above-described silica powder with a silicon compound having reducing properties so as to form a layer of the silicon compound on the surface of the silica.
Examples of the reducing silicon compounds used herein include, in addition to the carbon-functional silane monomers referred to above, polysilanes, polycarbosilanes, polysiloxanes and polysilazanes having silicon-silicon or silicon-hydrogen bonds. Of these, polysiloxanes with hydrogen atoms directly bonded to silicon atoms and polysilanes are especially preferred.
Exemplary silanes include polymeric compounds of the following general formula which have silicon-silicon bonds on the main chain.
(R2mR3nXpSi)qxe2x80x83xe2x80x83(2)
In formula (2), R2 and R3 are each independently a hydrogen or a substituted or unsubstituted monovalent hydrocarbon group such as an aliphatic, alicyclic or aromatic monovalent hydrocarbon group. Suitable aliphatic or alicyclic monovalent hydrocarbon groups are those having 1 to 12 carbons, and especially 1 to 6 carbons, for example, alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl and hexyl and cycloalkyl groups, such as cyclopentyl and cyclohexyl. Suitable aromatic monovalent hydrocarbon groups include those having 6 to 14 carbons, and preferably 6 to 10 carbons, such as phenyl, tolyl, xylyl, naphthyl and benzyl. Suitable substituted monovalent hydrocarbon groups include the aforementioned unsubstituted monovalent hydrocarbon groups on which some or all of the hydrogen atoms have been replaced with substituents such as halogen atoms, alkoxy groups, amino groups or aminoalkyl groups. Specific examples of such substituted monovalent hydrocarbon groups include monofluoromethyl, trifluoromethyl and m-dimethylaminophenyl.
In above formula (2), X may be the same type of group as R2, an alkoxy group, a halogen atom, an oxygen atom or a nitrogen atom. Suitable alkoxy groups include those having 1 to 4 carbons, such as methoxy, ethoxy and isopropoxy. Suitable halogen atoms include fluorine, chlorine and bromine. Of the above, it is generally preferable for X to be methoxy or ethoxy.
In formula (2), the letter m is from 0.1 to 1, and preferably from 0.5 to 1; n is from 0.1 to 1, and preferably from 0.5 to 1; and p is from 0 to 0.5, and preferably from 0 to 0.2. The sum m+n+p is from 1 to 2.5, and preferably from 1.5 to 2. In addition, q is an integer from 2 to 100,000, and preferably from 10 to 10,000.
The silicon compound having hydrogen atoms bonded directly to silicon atoms (Sixe2x80x94H groups) may be any organohydrogenpolysiloxane with hydrogen atoms bond directly to silicon atoms, although the use of a polysiloxane of general formula (3) below having pendant Sixe2x80x94H groups and having Sixe2x80x94Oxe2x80x94Si bonds on the backbone is preferred.
(R4rR5sHtSiOu)vxe2x80x83xe2x80x83(3)
In formula (3), R4 and R5 are each independently a hydrogen, a substituted or unsubstituted monovalent hydrocarbon group, an alkoxy group or a halogen atom. Monovalent hydrocarbon groups that may be used include aliphatic, alicyclic and aromatic monovalent hydrocarbon groups. Suitable aliphatic or alicyclic monovalent hydrocarbon groups are those having 1 to 12 carbons, and especially 1 to 6 carbons, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl and hexyl and cycloalkyl groups such as cyclopentyl and cyclohexyl. Suitable aromatic monovalent hydrocarbon groups include those having 6 to 14 carbons, and preferably 6 to 10 carbons, such as phenyl, tolyl, xylyl, naphthyl and benzyl. Suitable substituted aliphatic, alicyclic and aromatic monovalent hydrocarbon groups include the aforementioned unsubstituted monovalent hydrocarbon groups on which some or all of the hydrogen atoms have been replaced with substituents such as halogen atoms, alkoxy groups, amino groups or aminoalkyl groups. Specific examples of such substituted monovalent hydrocarbon groups include monofluoromethyl, trifluoromethyl and m-dimethylaminophenyl. Suitable alkoxy groups include those having 1 to 4 carbons, such as methoxy, ethoxy and isopropoxy. Suitable halogen atoms include fluorine, chlorine and bromine. The use of methoxy and ethoxy as R4 and R5 is generally preferred.
In formula (3), the letter r is from 0.1 to 1, and preferably from 0.5 to 1; s is from 0.1 to 1, and preferably from 0.5 to 1; and t is from 0.01 to 1, and preferably from 0.1 to 1. The sum r+s+t is from 2 to 2.5, and preferably from 2 to 2.2. In addition, u is from 1 to 1.5; and v is an integer from 2 to 100,000, and preferably from 10 to 10,000.
Step 1 of forming a layer of the silicon compound on the silica surface may be carried out by dissolving the silicon compound in an organic solvent, adding and mixing the silica powder into the solution, then removing the organic solvent so as to leave a layer of the silicon compound on the surface of the silica.
Suitable examples of the organic solvent which may be used to dissolve the silicon compound in this step include aromatic hydrocarbon solvents such as benzene, toluene and xylene; aliphatic hydrocarbon solvents such as hexane, octane and cyclohexane; ether solvents such as tetrahydrofuran and dibutyl ether; ester solvents such as ethyl acetate; polar aprotic solvents such as dimethyl-formamide, dimethylsulfoxide and hexamethylphosphoric triamide; and nitromethane and acetonitrile.
The silicon compound-containing solution typically has a concentration of from 0.01 to 30% by weight, and preferably 1 to 10% by weight. At less than 0.01% by weight, the large amount of solvent required tends to increase costs excessively. On the other hand, at a concentration above 30%, the silicon compound may not form sufficiently as a layer over the entire surface of the powder.
Preferred methods for treating the silica powder with the silicon compound-containing organic solvent solution include a stirring technique in which the silicon compound is dissolved in a solvent to form a dilute state which is then mixed with the silica powder, and the resulting slurry is stirred with an agitating element in a vessel to effect dispersion and contact; and a spray technique in which the same slurry is dispersed in a stream of air to effect instantaneous drying.
In the above-described treatment step, the organic solvent is driven off by raising the temperature and/or applying a vacuum. It is generally effective to carry out drying by stirring at a temperature above the boiling point of the solvent, such as a temperature of about 40 to 200xc2x0 C. under a vacuum of 1 to 100 mmHg.
Following treatment, by placing the treated powder in a drying atmosphere or heating it for a while at about 40 to 200xc2x0 C. under reduced pressure, the solvent can be effectively removed. Drying of the treated powder in this manner yields a silicon compound-treated silica powder.
The silicon compound layer has a thickness in a range of preferably 0.001 to 1 xcexcm, and especially 0.01 to 0.1 xcexcm. At less than 0.001 xcexcm, the silica may not be completely covered, as a result of which improperly plated areas may arise in a later step. On the other hand, at a thickness greater than 1 xcexcm, the amount of silicon compound used may become so large as to entail excessive costs.
Silicon compound treatment renders the silica powder hydrophobic. The treated silica powder thus has a lowered affinity with the solvent used to dissolve the metal salt and may not disperse well therein, lowering the efficiency of the metal salt reduction reaction. Such a decline in the efficiency of the metal salt reduction reaction can be countered by the addition of a surfactant. Preferred surfactants are those which lower the surface tension without inducing the formation of foam. Illustrative examples of surfactants suitable for this purpose include the nonionic surfactants Surfynol 104, 420 and 504, all products of Nisshin Chemical Industry Co., Ltd.
In the next step (Step 2), the powder obtained in step 1 by forming a silicon compound layer on the silica surface is treated with a solution containing a metal salt of a metal having a standard redox potential of at least 0.54 V to induce the deposition of a colloid of the metal onto the silicon compound layer. This step involves contacting the surface of the silicon compound-treated powder with a metal salt-containing solution. The reducing action of the silicon compound causes a colloid of the metal to form on the surface of the silicon compound film, resulting in the formation of a metal film.
Suitable examples of the salts of a metal having a standard redox potential of at least 0.54 V which may be used in this step include salts of gold (standard redox potential, 1.50 V), palladium (standard redox potential, 0.99 V) or silver (standard redox potential, 0.80 V). Salts of metals having a standard redox potential lower than 0.54 V, such as copper (standard redox potential, 0.34 V) and nickel (standard redox potential, 0.25 V), are not easily reduced by the silicon compound.
The gold salt is a salt containing Au+ or Au3+, such as NaAuCl4, NaAu(CN)2 and NaAu(CN)4. The palladium salt is a salt containing Pd2+, which can generally be expressed in the form PdZ2, wherein Z is a halogen such as chlorine, bromine or iodine, acetate, trifluoroacetate, acetylacetonate, carbonate, perchlorate, nitrate, sulfate or oxide. Specific examples of suitable palladium salts include PdCl2, PdBr2, PdI2, Pd(OCOCH3)2, Pd(OCOCF3)2, PdSO4, Pd(NO3)2 and PdO. The silver salt is a salt which dissolves in the solvent to form Ag+ ions, and can generally be represented as AgZ, wherein Z is perchlorate, borate, phosphate or sulfonate. Specific examples of suitable silver salts include AgBF4, AgClO4, AgPF6, AgBPh4 (wherein xe2x80x9cPhxe2x80x9d stands for phenyl), Ag(CF3SO3) and AgNO3.
Suitable examples of the solvent used to dissolve the metal salt include water, ketones such as acetone and methyl ethyl ketone, alcohols such as methanol and ethanol, and polar aprotic solvents such as dimethylformamide, dimethylsulfoxide and hexamethylphosphoric triamide. Of these, water is preferred.
The concentration of the metal salt varies according to the solvent used to dissolve the salt, although a concentration within a range of from 0.01% by weight to the saturation concentration is preferred. At less than 0.1% by weight, the plating catalyst may not be adequately effective, whereas a concentration greater than that needed to give a saturated solution often results in the precipitation of solid salt. When the solvent is water, the metal salt concentration is preferably within a range of 0.01 to 20%, and especially 0.1 to 5%. The silicon compound-treated powder is generally immersed in the metal salt solution at a temperature within a range of room temperature to 70xc2x0 C. for a period of about 0.1 to 120 minutes, and preferably about 1 to 15 minutes, thereby forming a metal colloid-treated powder.
Step 2 is preferably carried out by first contacting the silicon compound-treated powder with a water-diluted surfactant, then contacting the powder with the above-described metal salt solution. This makes it possible to prevent the decrease in efficiency of the metal salt reducing reaction that would otherwise arise from the hydrophobizing effect of silicon compound treatment in step 1 on the silica surface, which reduces affinity of the powder to the metal salt-dissolving solvent and thus discourages dispersion of the powder in the solution. Thus, by first contacting the silicon compound-treated powder with a water-diluted surfactant, the silicon compound-treated powder can be easily and rapidly dispersed in the metal salt-containing solution.
The surfactant used for this purpose may be an anionic surfactant, cationic surfactant, amphoteric surfactant or nonionic surfactant.
Suitable anionic surfactants include sulfonate, sulfate, carboxylate and phosphate surfactants. Suitable cationic surfactants include ammonium salt, alkylamine salt and pyridinium salt surfactants. Suitable amphoteric surfactants include betaine, aminocarboxylic acid and amine oxide surfactants. Suitable nonionic surfactants include ether, ester and silicone surfactants.
Specific types of anionic surfactants that may be used include alkylbenzenesulfonates, sulfosuccinates, alkyl polyoxyethylenesulfates, alkyl phosphates, and long-chain fatty acid soaps. Cationic surfactants that may be used include alkyltrimethylammonium chloride, dialkyldimethylammonium chloride and alkylpyridinium chloride. Amphoteric surfactants that may be used include betaine sulfonates and betaine aminocarboxylates. Nonionic surfactants that may be used include polyoxyethylene alkyl ethers, polyoxyethylene fatty acid esters and polyoxyalkylene-modified polysiloxanes. Use can also be made of commercially available aqueous solutions of surfactant mixtures, such as those manufactured by Lion Corporation under the trade name Mama Lemon.
If necessary, any surfactant as described above may be used in an amount of 0.0001 to 10 parts, preferably 0.001 to 1 part, and especially 0.01 to 0.5 part, per 100 parts of the metal salt solution.
Following metal salt treatment, excess metal salt that has not been supported on the powder is removed by treatment with the same type of solvent as described above, but containing no metal salt, after which the powder is dried to remove unwanted solvent. Drying is preferably carried out at a temperature of 0 to 150xc2x0 C., either under atmospheric pressure or in a vacuum.
In Step 3, electroless nickel plating is carried out using as the catalyst the metal colloid that has been deposited on the surface of the powder particles, thereby forming a layer of metallic nickel on the surface of the silicon compound layer.
The electroless nickel plating solution generally contains a water-soluble metallic nickel salt such as nickel sulfate or nickel chloride, a reducing agent such as sodium hypophosphite, hydrazine or sodium borohydride, a pH modifier such as sodium acetate, and a complexing agent such as phenylenediamine or sodium potassium tartrate. A commercial electroless nickel plating solution may be used for the purposes of the invention.
The electroless nickel plating process may be carried out in accordance with normal practice. That is, use may be made of a batch process in which plating is effected by adding the powder to the electroless plating solution, or a dropwise addition process in which the plating solution is gradually added to the powder dispersed in water (see Dodensei fira no kaihatsu to oyo (Development and use of electrically conductive fillers), Gijutsu Joho Kyokai, 1994, p. 182). Whichever method is used, it is important to control the plating rate so as to prevent agglomeration of the particles and obtain a uniform nickel film that adheres well. However, it can sometimes be difficult to obtain such nickel-coated silica. For example, in powders having a high specific surface area to allow plating reaction to be very active, the plating reaction tends to start abruptly and becomes impossible to control. Conversely, the start of plating may be delayed due to the influence of ambient oxygen, in which case nickel plating takes a long time and is less likely to yield a uniformly plated powder.
Accordingly, it is advantageous to effect nickel plating on silica by the following process. The nickel plating solution is divided into an aqueous solution containing, for example, a reducing agent, pH modifier and complexing agent, and an aqueous solution of the nickel salt. The silica is dispersed in the aqueous solution containing the reducing agent, pH modifier and complexing agent, and is held at the optimal temperature for nickel plating. The nickel salt solution is then added by entrainment with a gas to the reducing agent-containing aqueous solution in which the silica has been dispersed. We have found this process to be very effective for preparing nickel-coated silica free of agglomeration. The gas used to entrain the nickel salt solution has the effect of rapidly and uniformly dispersing this solution in the aqueous solution containing the reducing agent, pH modifier and complexing agent, thereby plating the surface of the powder with nickel.
The introduction of gas sometimes lowers the plating efficiency due to foaming, but this can be prevented by the addition of an antifoaming surfactant. The surfactant used for this purpose should be one which has an antifoaming action that lowers the surface tension. Suitable surfactants include polyether-modified silicone surfactants such as KS-538 (made by Shin-Etsu Chemical Co., Ltd.).
In electroless nickel plating, the oxygen concentration in the plating solution exerts an influence on the precipitation of nickel. A large amount of dissolved oxygen interferes with nickel deposition, either by causing the colloidal palladium serving as the plating catalyst nuclei to oxidize to palladium cations, which then dissolve into the plating solution, or by oxidizing the surface of the nickel that has already been deposited. On the other hand, the presence of too little dissolved oxygen lowers the stability of the plating solution, causing nickel to deposit in places other than on the silica and resulting in the formation of fine nickel dust or bumpy deposits. It is therefore preferable for the amount of dissolved oxygen in the plating solution to be maintained within a range of 1 to 20 ppm. The presence of more than 20 ppm of oxygen may result in a decline in the plating rate and the emergence of unplated areas, whereas less than 1 ppm may lead to the formation of bumpy deposits.
The gas used for the above purpose is preferably a mixture of an oxygen-containing gas such as air and an inert gas such as argon or nitrogen. In the powder plating process, plating is often slow to begin, but once it does begin, reaction runaway may occur. One effective way to keep this from happening is to use nitrogen at the beginning and, after confirming that the nickel plating reaction has begun, to subsequently switch over to air. Typically, the plating temperature is from 35 to 120xc2x0 C., and the catalyst contact time is from 1 minute to 16 hours. Treatment conditions of 40 to 85xc2x0 C. and 10 to 60 minutes are preferred.
Step 4, which follows the above-described electroless nickel plating step, is to carry out gold plating so as to form a layer of gold on the deposited layer of nickel.
The gold plating solution may be an electroplating solution or an electroless plating solution. Use may be made of a solution of known composition or a commercial product, although an electroless gold plating solution is preferred. The gold plating process may be carried in accordance with the conventional plating process described above. It is effective to carry out gold plating after using a dilute acid to remove the surface of the nickel that has been passivated by oxidation. The gold plating temperature and catalyst contact time are the same as in nickel plating.
At the end of the plating process, the treated powder should be rinsed with water to remove unwanted surfactant. The powder obtained from this process is a metal-plated silica powder having a silica/silicon compound/nickel/gold four-layer construction.
The nickel layer has a thickness within a range of preferably 0.01 to 10.0 xcexcm, and especially 0.1 to 2.0 xcexcm. At less than 0.01 xcexcm, it may be difficult to obtain a metal-plated silica powder in which the silica is completely covered, and which has sufficient hardness and strength. On the other hand, at a thickness greater than 10.0 xcexcm, the amount of nickel becomes large and the specific gravity high, which may make the metal-plated silica powder too expensive for compounding in the silicone rubber composition of the invention.
The gold layer has a thickness within a range of preferably 0.001 to 1.0 xcexcm, and especially 0.01 to 0.1 xcexcm. At less than 0.001 xcexcm, the resistivity becomes so high that compounding of the metal-plated silica powder may fail to impart the inventive composition with sufficient electrical conductivity. On the other hand, at a thickness greater than 1.0 xcexcm, the amount of gold used may become so great as to be prohibitively expensive.
Finally, it is advantageous for the metal-plated silica powder obtained by the process described above to be heat treated in the presence of an inert gas such as N2 or a reducing gas such as H2 and at a temperature of at least 200xc2x0 C. The treatment conditions are typically a temperature of 200 to 900xc2x0 C., and a treatment time of 1 minute to 24 hours, although treatment conditions of 250 to 500xc2x0 C. and 30 minutes to 4 hours are preferred. Such heat treatment converts the silicon compound situated between the silica powder and the metal to a ceramic, which has a higher heat resistance, insulating properties and adhesion. Carrying out heat treatment in a reducing atmosphere such as hydrogen decreases the amount of oxide in the metal and transforms the silicon compound to a more stable structure, resulting in strong bonding between the silica and the metal, and ultimately giving a powder having a high electrical conductivity.
Heat treatment in such a hydrogen reducing environment converts the silicon compound primarily to a silicon carbide ceramic. That is, the above-described high temperature treatment converts some or all of the silicon compound between the silica powder and the metal film to a ceramic, which has a higher heat resistance, insulating properties and adhesion than the silicon compound.
The metal-plated silica powder may be imparted with affinity to silicone by administering hydrophobic treatment using, for example, any of various alkoxysilanes, titanate treatment agents, carbon-functional silanes, silazanes, or silanol-containing oligomeric siloxanes.
The inventive composition may include, together with component (C), another electrically conductive material, examples of which include known conductive inorganic substances such as conductive carbon black, conductive zinc oxide and conductive titanium oxide; and/or an extender, examples of which include fillers such as silicone rubber powder, red iron oxide and calcium carbonate.
The amount of component (C) included in the inventive composition is 30 to 700 parts by weight, and preferably 50 to 600 parts by weight, per 100 parts by weight of the organopolysiloxane serving as component A. The use of less than 30 parts by weight may fail to provide the desired low electrical resistance, whereas an amount greater than 700 parts by weight may compromise the processability of the composition to such an extent as to render injection molding impossible.
Component (D) is an adhesive aid or tackifier for imparting adhesion to the composition of the invention. The adhesive aid is not critical as long as it can impart adhesion. Useful adhesive aids include silane coupling agents and titanium coupling agents although preferred adhesive aids are compounds containing at least one alkoxy and/or epoxy group in a molecule; organosilicon compounds having in a molecule at least two different groups selected from among SiH, epoxy, alkoxy and alkenyloxy groups; and organosilicon compounds having in a molecule at least one SiH group and at least one group containing an aromatic ring and/or a carbonyl group.
Representative of these adhesive aids are organosilicon compounds of the following classes (i), (ii) and (iii), which may be used alone or in admixture of any.
(i) Organosilicon compounds having at least one, preferably at least two, more preferably at least three, alkoxy group attached to a silicon atom in a molecule, represented by the following average compositional formula (4).
xe2x80x83R6bR7c(OR8)dSiO(4xe2x88x92bxe2x88x92cxe2x88x92d)/2xe2x80x83xe2x80x83(4)
Herein, R6 is a substituted or unsubstituted monovalent hydrocarbon group, R7 is a monovalent organic group having a functional group capable of addition reaction or condensation reaction, R8 is a substituted or unsubstituted monovalent hydrocarbon group, b, c and d are numbers satisfying 0xe2x89xa6bxe2x89xa63, 0xe2x89xa6cxe2x89xa63, 0 less than dxe2x89xa64, and 0 less than b+c+dxe2x89xa64.
(ii) Organosilicon compounds such as organohydrogenpolysiloxanes having at least one, preferably at least two, more preferably at least three, SiH group in a molecule, represented by the following average compositional formula (5).
R9eHfSiO(4xe2x88x92exe2x88x92f)/2xe2x80x83xe2x80x83(5)
Herein, R9 is a substituted monovalent hydrocarbon group, e and f are numbers satisfying 0 less than exe2x89xa63, 0 less than fxe2x89xa63, and 1xe2x89xa6e+f less than 4.
(iii) Organosilicon compounds having in a molecule at least one, preferably at least two, more preferably at least three, SiH group and at least one, preferably 1 to 4, substituted or unsubstituted, divalent or trivalent aromatic ring and/or carbonyl group.
Referring to formula (4), R6 stands for substituted or unsubstituted monovalent hydrocarbon groups, preferably of 1 to 8 carbon atoms and free of aliphatic unsaturation, for example, alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl, cyclohexyl and octyl, aryl groups such as phenyl, tolyl and xylyl, aralkyl groups such as benzyl and phenylethyl, and halo- and cyano-substituted hydrocarbon groups in which some or all of the hydrogen atoms on the foregoing hydrocarbon groups are replaced by halogen atoms or cyano groups, such as chloromethyl, bromoethyl, 3,3,3-trifluoropropyl 3-chloropropyl and cyanoethyl.
R7 stands for monovalent organic groups having a functional group capable of addition reaction or condensation reaction, examples of which include groups of the formula: CH2xe2x95x90CRxe2x80x94COO(CH2)nxe2x80x94 wherein R is hydrogen or methyl and n is an integer of 1 to 8, such as (meth)acryloxypropyl; alkenyl groups of 2 to 6 carbon atoms such as vinyl, allyl, propenyl, isopropenyl and butenyl; groups of the formula: H2Nxe2x80x94(CH2)nxe2x80x94 wherein n is an integer of 1 to 8, such as aminopropyl; groups of the formula: HSxe2x80x94(CH2)nxe2x80x94 wherein n is an integer of 1 to 8, such as mercaptopropyl; and groups of the formula: Gxe2x80x94(CH2)nxe2x80x94 wherein G is an epoxy-containing group as shown below and n is an integer of 1 to 8, such as glycidyloxypropyl. 
R8 stands for substituted or unsubstituted monovalent hydrocarbon groups, preferably of 1 to 8 carbon atoms. Examples are the same as described for R1, although preferred examples are alkyl and alkoxy-substituted alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, octyl, methoxyethyl, methoxymethyl, ethoxyethyl and ethoxymethyl.
Subscripts b, c and d are numbers satisfying 0xe2x89xa6bxe2x89xa63, preferably 0xe2x89xa6bxe2x89xa62, 0xe2x89xa6cxe2x89xa63, preferably 0 less than c less than 2, 0 less than dxe2x89xa64, preferably 1xe2x89xa6d less than 4, more preferably 2xe2x89xa6dxe2x89xa63, and 0 less than b+c+dxe2x89xa64, preferably 1xe2x89xa6b+c+dxe2x89xa63.
Specifically, the compounds of formula (4) are classified into tetra(organooxy)silanes such as tetraalkoxysilanes, organotri(organooxy)silanes such as organotrialkoxysilanes, diorganodi(organooxy)silanes such as diorganodialkoxysilanes, and organooxy group-bearing siloxanes including organoalkoxysiloxanes having two to several silicon atoms and alkoxysiloxanes having two to several silicon atoms, available as partial hydrolytic condensates of the foregoing silane compounds. The organooxy group-bearing siloxanes including organoalkoxysiloxanes and alkoxysiloxanes may have linear, cyclic, branched or network structures and may be either homopolymers or copolymers although those siloxanes which are liquid at room temperature are often preferred.
Illustrative examples of the compounds of formula (4) include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(xcex2-methoxyethoxy)silane, xcex3-methacryloxypropyl-trimethoxysilane, xcex3-methacryloxypropyltriethoxysilane, xcex3-acryloxypropyltrimethoxysilane, allyltrimethoxysilane, allyltriethoxysilane, xcex3-mercaptopropyltrimethoxysilane, N-xcex2-(aminoethyl)-xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyl-triethoxysilane, mercaptoethyltriethoxysilane, xcex3-glycidoxypropyltrimethoxysilane, xcex2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, and partial hydrolytic condensates of one or more of the foregoing silanes.
Referring to formula (5), R9 is typically an epoxy group-substituted hydrocarbon group or an alkoxy or alkenyloxy group-containing group.
The epoxy group-substituted hydrocarbon groups are such that an epoxy group is attached to a silicon atom through a divalent hydrocarbon group such as alkylene as represented by the above formula: G-(CH2)nxe2x80x94, such as, for example, xcex3-glycidoxypropyl and xcex2-(3,4-epoxycyclohexyl)ethyl. In the alkoxy or alkenyloxy group-containing hydrocarbon groups, exemplary alkoxy groups include alkoxy groups of 1 to 4 carbon atoms such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, tert-butoxy, methoxymethoxy, methoxyethoxy, ethoxymethoxy, and ethoxyethoxy, and exemplary alkenyloxy groups include alkenyloxy groups of 2 to 8 carbon atoms such as vinyloxy, propenyloxy, and allyloxy. The preferred alkoxy or alkenyloxy group-containing groups have the formula:
xe2x80x83xe2x80x94Xxe2x80x94SiYyZz
wherein X is a C1-12 alkylene group which may be separated by one or more COO, CONH, OCONH, CO or oxygen atoms, Y is alkoxy or alkenyloxy group, Z is a C1-6 alkyl group or phenyl group, y is 1, 2 or 3, and z is 0, 1 or 2, with y+z=3. Illustrative examples are xcex3-methyldimethoxysilylpropyl, xcex3-dimethylmethoxysilylpropyl, xcex3-trimethoxysilylpropyl, and xcex3-triethoxysilylpropyl.
The hydrogen atom to be attached to a silicon atom (to form a SiH group) may be attached to a silicon atom at the end of the molecular chain, that is, a silicon atom in a monofunctional siloxane unit represented by xe2x95x90SiO1/2, although it is preferably attached to a silicon atom midway the molecular chain, that is, a silicon atom in a difunctional siloxane unit represented by xe2x95x90SiO2/2.
In formula (5), subscripts e and f are numbers satisfying 0 less than exe2x89xa63, preferably 1xe2x89xa6exe2x89xa62, 0 less than fxe2x89xa63, preferably 0.1xe2x89xa6fxe2x89xa61, and 1xe2x89xa6e+f less than 4, preferably 1.6xe2x89xa6e+fxe2x89xa63. The organosilicon compound or organopolysiloxane of formula (5) may be linear, branched or cyclic.
Preferably the organosilicon compound of formula (5) has a viscosity of about 1 to 1,000 centipoise (cp) at 25xc2x0 C. and especially about 5 to 200 cp at 25xc2x0 C.
Illustrative, non-limiting, examples of the organosilicon compound of formula (5) are given below. 
In the formulae, x is an integer of 1 to 50, y is an integer of 1 to 100, and z is an integer of 1 to 100.
Class (iii) consists of organosilicon compounds having in a molecule at least one SiH group and at least one, preferably 1 to 4, substituted or unsubstituted, di- or trivalent aromatic ring such as phenylene skeleton as a mono- to trivalent group attached to a silicon atom forming the siloxane skeleton and/or carbonyl group. It is noted that the mono- to trivalent group having an aromatic ring may be one having an ester structure (xe2x80x94COOxe2x80x94) or ether oxygen (xe2x80x94Oxe2x80x94).
Examples of the mono- to trivalent group having a di- or trivalent aromatic ring or carbonyl group include groups having the following partial structure. 
Herein, n is an integer of 1 to 6.
Illustrative examples of the organosilicon compounds (iii) are given below. 
Of the organosilicon compounds of classes (ii) and (iii), those compounds having mono- to trivalent cyclic methylhydrogensiloxane structures as shown below as a partial structure within their molecule are preferred. 
The adhesive aid (D) is added in an amount of 0.1 to 20 parts by weight per 100 parts by weight of the organopolysiloxane (A). Less than 0.1 part of the adhesive aid fails to impart the desired self adhesiveness whereas more than 20 parts of the adhesive aid can adversely affect the physical properties of the rubber composition. The preferred amount of adhesive aid is 0.2 to 15 parts by weight.
Component (E) of the inventive silicone rubber composition is a curing agent. In the practice of the invention, the components described above are uniformly blended using a rubber compounding apparatus such as a two-roll mill, a Banbury mixer or a dough mixer (kneader). If necessary, the blend is heat-treated at atmospheric pressure or reduced pressure. The resulting electrically conductive liquid silicone rubber composition is then cured by adding and blending therein a suitable curing agent, thereby giving a rubber elastomer having a stable, high electrical conductivity. The curing process employed for this purpose may be an organic peroxide-based curing process or an addition curing process involving the use of an addition crosslinking agent and a catalyst.
In the organic peroxide-based curing process, use is made of organic peroxide curing agents, preferred examples of which include benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide, 2,4-dicumyl peroxide, 2,5-dimethylbis(2,5-t-butylperoxy)hexane, di-t-butyl peroxide and t-butyl perbenzoate.
The organic peroxide may be used singly or as a combination of two or more thereof. The amount of organic peroxide addition is preferably 0.1 to 10 parts by weight, and especially 0.2 to 5 parts by weight, per 100 parts by weight of the organopolysiloxane (A). The addition of too little organic peroxide may result in inadequate crosslinkage, whereas the addition of more than the above range may fail to provide any additional improvement in the curing speed.
Curing processes which involve an addition reaction use as the curing agent an addition reaction-type curing agent composed of an organohydrogenpolysiloxane having at least two SiH groups in a molecule and a platinum metal-based addition reaction catalyst. The preferred organohydrogenpolysiloxane used herein has the following general formula (6).
R10gHhSiO(4xe2x88x92gxe2x88x92h)/2xe2x80x83xe2x80x83(6)
In formula (6), R10 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, and is exemplified by the same groups as mentioned earlier for R1 in formula (1). Groups free of aliphatic unsaturation are preferred, with methyl being most preferred. The letter g is a positive number from 0.7 to 2.1, and preferably from 1 to 2; h is a positive number from 0.002 to 1, and preferably from 0.01 to 0.9. The sum g+h is a positive number from 0.8 to 3, and preferably from 1.5 to 2.8.
The organohydrogenpolysiloxane acts as a crosslinking agent for component (A) in the presence of an addition reaction catalyst. Each molecule has at least two, and preferably at least three, hydrogen atoms bonded to silicon atoms. These Sixe2x80x94H groups may be positioned at the ends of the molecular chain, in the middle of the molecular chain, or in both places.
Illustrative examples of such organohydrogenpolysiloxanes include methylhydrogenpolysiloxane capped at both ends with trimethylsiloxy groups, dimethylsiloxane-methylhydrogenpolysiloxane copolymers capped at both ends with trimethylsiloxy groups, dimethylsiloxane capped at both ends with dimethylhydrogensiloxy groups, dimethylsiloxane capped at both ends with dimethylhydrogenpolysiloxy groups, dimethylsiloxane-methylhydrogenpolysiloxane copolymers capped at both ends with dimethylhydrogenpolysiloxy groups, methylhydrogenpolysiloxane-diphenylsiloxane-dimethylsiloxane copolymers capped at both ends with trimethylsiloxy groups, copolymers composed of (CH3)2HSiO1/2 units and SiO4/2 units and copolymers composed of (CH3)2HSiO1/2 units, SiO4/2 units and (C6H5)SiO3/2 units.
The organohydrogenpolysiloxane of above formula (6) has a molecular structure which may be linear, branched, cyclic or a three-dimensional network structure. The molecular weight is not critical, although the compound is preferably liquid at room temperature. The viscosity at 25xc2x0 C. is in a range of preferably 0.1 to 1,000 cp, and especially 0.5 to 500 cp. Such an organohydrogenpolysiloxane may be prepared by a suitable known process.
The foregoing organohydrogenpolysiloxane is included in the silicone rubber composition in an amount of preferably 0.1 to 100 parts by weight, more preferably 0.3 to 50 parts by weight, and most preferably 0.5 to 20 parts by weight, per 100 parts by weight of the organopolysiloxane (A). Addition of too little organohydrogenpolysiloxane may result in inadequate crosslinkage, whereas addition in an amount greater than the above range may fail to provide the desired increase in curing rate.
It is particularly advantageous for the organohydrogenpolysiloxane to be added in an amount such that the molar ratio of hydrogen atoms bonded to silicon atoms (SiH groups) therein, relative to alkenyl groups in the organopolysiloxane (A), is from 0.3 to 20, and especially from 0.8 to 3.
The addition reaction catalyst may be selected from platinum catalysts such as platinum black, platinic chloride, the reaction product of chloroplatinic acid with a monohydric alcohol, a chloroplatinic acid-olefin complex and platinum bisacetoacetate, palladium catalysts and rhodium catalysts. The addition reaction catalyst is preferably included in an amount of about 0.1 to 2,000 ppm, and especially about 1 to 500 ppm, calculated as the platinum, palladium or rhodium metal, based on the weight of the organopolysiloxane (A).
Insofar as the desired effects of the invention are attainable, the silicone rubber composition may also have added thereto one or more addition reaction regulators selected from among triallyl isocyanate, alkyl maleates, acetylene alcohols such as ethynylcyclohexanol, modified silanes and siloxanes, hydroperoxide, tetramethylethylenediamine, benzotriazole, and mixtures thereof, as an optional component other than the above-described main components.
In addition to the components described above, the electrically conductive silicone rubber compositions of the invention may also include a non-reactive silicone oil to impart water repellency and sealing and slip properties. Illustrative examples of suitable silicone oils include linear dimethylpolysiloxane, diphenylpolysiloxane, phenylmethylpolysiloxane and hydroxyl group-bearing organopolysiloxane. If necessary, the composition may also include various additives, such as metal oxides, e.g., titanium oxide, iron oxide, cerium oxide, vanadium oxide, cobalt oxide, chromium oxide, and manganese oxide. Additional constituents that may be included, to the extent that the objects and effects of the invention are not compromised, include pigments, heat insulators, flame retardants, plasticizers and reaction regulators.
The electrically conductive liquid silicone rubber composition of the invention may be prepared by uniformly mixing the above-described components (A) to (E) and other, optional, components at room temperature. However, if necessary, it is possible to heat-treat all the components other than component (E) in a suitable apparatus such as a planetary mixer or kneader at a temperature of about 100 to 200xc2x0 C. for a period of about 30 minutes to 4 hours, especially about 2 to 4 hours, and subsequently mix in component (E) to effect curing and molding. The molding method may be selected as appropriate for the viscosity of the mixture. Suitable molding methods include casting, compression molding, injection molding and transfer molding. Curing conditions of about 80 to 200xc2x0 C. for about 3 minutes to 3 hours are generally preferred.
When the inventive composition is liquid, injection molding is advantageously employed. For injection molding purposes, it is preferable for the composition to have a viscosity of about 300 to 100,000 poises at 25xc2x0 C., and especially about 1,000 to 5,000 poises at 25xc2x0 C. When the inventive composition is of millable type, the advantageous molding method is compression molding.
The electrically conductive silicone rubber compositions of the invention are applied and cured to substrates of metals such as stainless steel, steel, aluminum and copper and resins such as polycarbonate (PC), polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) without a need for application of primers. The cured coatings of the silicone rubber compositions are directly and integrally joined to the substrates to give articles which are useful as parts in various electrical equipment, electrical contacts, and sealants.
There have been described conductive silicone rubber compositions which have a low volumetric resistivity, a stable electrical resistance and can be cured into silicone rubber. The resulting silicone rubber is useful as electrical components and electrical contacts employed in high-conductivity applications.